A bracket of the type involving a base plate having a top and bottom side adapted to be attached to a tooth, two spaced apart wings arranged on the top side with an elongate slot extending therethrough for receiving an arch wire, and a closing spring made of a band-like material, anchored to the structure and movable between an open position in which the slot is freely accessible in an area between the wing sections and a closing position in which the free leg of the spring covers the slot is known, for instance, from U.S. Pat. No. 5,562,444. In the known bracket, an elongate leg is formed at the end of the arc of the closing spring with the free end of the leg being positioned in a broad groove formed on the bottom side of the gingival wing sections covered by the arc of the spring. This groove has a great opening angle which allows pivoting of the closing spring around the gingival wing sections between an open and a closed position, wherein the apex of the groove forms a pivot bearing together with the edge of the free end of the leg arranged in the groove. In the opening and closing positions, the spring holds on to the gingival wing sections covered by the spring by its inherent spring force by clamping effect.
However, it turned out in practice that when carelessly opening the spring the spring's elasticity limit can be exceeded so that the retention force by which it usually holds on to the gingival wing sections of the bracket structure is weakened. It can therefore easily get lost since the clamping force of the spring at the wing sections is the only means by which the spring holds onto its place.
In the above-mentioned reference, another bracket is described as including a spring which has an elongation at the end of the arc which does not cover the slot, with the elongation being bifurcated into two legs that are received by channels which extend laterally in the bracket structure below the slot and transversely thereto. In this bracket, there is also the risk of bending the spring too far, since only an arc of approximately 180° arc length is available for the bending process when opening the spring.
A bracket similar to the last mentioned bracket is known from U.S. Pat. No. 4,492,573, in which the bracket comprises an essentially U-shaped bent closing spring, the entirety of which, when opening the slot, can be moved transversely to the slot.
U.S. Pat. No. 4,144,642 also describes a bracket having a clip spring which can be shifted transversely with respect to the slot when opening and closing the slot for receiving the arch wire.
All the above-mentioned brackets have springs which, if they exert a force onto an arch wire inserted into the slot at all, exert a force directed towards the bottom of the slot, i.e., their springs press the arch wire towards the bottom of the slot. The spring force for each of these brackets is determined by the rigidity of the spring material and the effective spring length. The effective spring length is relatively great in all of these examples since it corresponds to the distance between the clamping location of the spring and the engagement location at the arch wire. Only when the spring rigidity is sufficiently great is sufficient closing force of the spring generated, which in turn enlarges the risk of a plastic deformation of the spring when moving it into the opening position.
Furthermore, a closing spring which only presses the arch wire on the bottom of the slot is less suitable for obtaining a torque which turns a treated tooth around an axis which extends through the level of the crown of the tooth.